The present invention relates to spread spectrum global navigation system receivers such as global positioning system (GPS) receivers. More particularly, the present invention relates to differential GPS (DGPS) base stations and receivers which implement methods of carrier smoothing code pseudorange measurements.
Global navigational satellite systems (GNSS) are known and include the global positioning system (GPS) and the Russian global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are used for navigation and positioning applications. In the GPS navigational system, GPS receivers receive satellite positioning signals from a set of up to 32 satellites deployed in 12-hour orbits about earth and dispersed in six orbital planes at an altitude of 10,900 nautical miles. Each GPS satellite continuously transmits two spread spectrum, L-band signals: an L1 signal having a frequency fL1 of 1575.42 MHz, and an L2 signal having a frequency fL2 of 1227.6 MHz. The L1 signal from each satellite is modulated by two pseudo-random codes, the coarse acquisition (C/A) code and the P-code. The P-code is normally encrypted, with the encrypted version of the P-code referred to as the Y-code. The L2 signal from each satellite is modulated by the Y-code. The C/A code is available for non-military uses, while the P-code (Y-code) is reserved for military uses.
GPS navigational systems determine positions by timing how long it takes the coded radio GPS signal to reach the receiver from a particular satellite (e.g., the travel time). The receiver generates a set of codes identical to those codes (e.g., the Y-code or the C/A-code) transmitted by the satellites. To calculate the travel time, the receiver determines how far it has to shift its own codes to match the codes transmitted by the satellites. The determined travel times for each satellite are multiplied by the speed of light to determine the distances from the satellites to the receiver. By receiving GPS signals from four or more satellites, a receiver unit can accurately determine its position in three dimensions (e.g., longitude, latitude, and altitude). A conventional GPS receiver typically utilizes the fourth satellite to accommodate a timing offset between the clocks in the receiver and the clocks in the satellites. Additional satellite measurements can be used to improve the position solution.
Differential GPS (DGPS) utilizes a base station located at a known position and one or more remote GPS receivers. The base station receives GPS positioning signals from the satellites and calculates predicted measurements based upon the known base station location. Based upon differences between the predicted base station measurements and the actual measurements, the base station transmits corrections to the remote GPS receiver. The remote GPS receiver uses the corrections and received GPS satellite signals to calculate its position more accurately.
The smoothing of GPS code pseudorange measurements with carrier phase measurements is a well-established GPS signal processing technique. See for example U.S. Pat. No. 6,198,430 B1 issued on Mar. 6, 2001 to Hwang et al. and entitled ENHANCED DIFFERENTIAL GNSS CARRIER-SMOOTHED CODE PROCESSING USING DUAL FREQUENCY MEASUREMENTS, which is incorporated by reference in its entirety. Dual Frequency Smoothing and associated DGPS architecture are also known. One advantage of Dual Frequency Smoothing compared to conventional single frequency carrier smoothing of code pseudorange measurements is the elimination of the effects of ionospheric divergence in the processing. In the DGPS context, Dual Frequency Smoothing permits the measurement smoothing operations at the reference station and the airborne unit to be completely decoupled which enables longer smoothing on the ground to mitigate multipath and shorter smoothing in the airborne unit for fast convergence. This flexibility in designing the carrier smoothing in differential processing is one of the advantages of Dual Frequency Smoothed DGPS over single frequency code DGPS.
Generally, two types of Dual Frequency Smoothing have been considered: “Divergence-Free” (DF) smoothing and “Ionosphere-Free” (IF) smoothing. In IF smoothing, the ionospheric delay is removed from the smoothed pseudorange, whereas in DF smoothing the effects of the ionospheric divergence are removed from the smoothing, but the instantaneous ionospheric delay remains in the smoothed pseudorange. Two specific forms of DF smoothing involve the L1 and L2 pseudoranges. The DF pseudoranges have substantially smaller noise and multipath errors than the IF pseudoranges, and with differential processing the residual ionospheric delay in the DF pseudoranges largely cancels. Additionally, other combinations of dual frequency pseudorange and carrier phase measurements can be processed in a DF form, with some operational advantages for geometry-free carrier phase integer ambiguity resolution.
Conventionally, to implement various specific DGPS smoothing modes of operation (e.g., wide-lane processing, iono-free processing, etc.), mode specific processing at the base or reference station must be implemented to generate mode specific correction data for transmission to the remote GPS receiver. The GPS receiver then implements further mode specific processing using the mode specific correction data from the base station. This adds complexity to the base station processing, particularly if it is desired to be able to switch from one specific mode to another (e.g., from Wide-Lane processing to Iono-Free processing, etc.).